The present invention relates to printed thermoplastic food-packaging films, and more particularly to a food product enclosed within a package formed from a printed film having a radiation-cured varnish covering the printed image of the film.
Printed thermoplastic films are in wide use for food packaging. For example, printed thermoplastic films are used with the vertical form-fill-seal (VFFS) packaging process to package several types of food products—such as solid or particulate food products (e.g., fresh cut produce, shredded cheese, or frozen chicken wings and nuggets) and liquified foods (e.g., soups and beverages). In a typical VFFS packaging process, a tubular film is provided, for example, by longitudinally heat sealing a printed film to itself to form the tube. This longitudinal seal may be formed as a lap seal or a fin seal. The tube is then heat-sealed transversely at its lower end to form the bottom of a pouch. The food product to be packaged flows through a vertical fill line and into the pouch. After filling, the pouch is closed by transversely heat sealing the open, upper end of the pouch to form a sealed pouch. Typically, this top transverse seal severs the sealed pouch from the tubular film above it, while simultaneously forming the bottom transverse seal of the next pouch.
An image that is printed on the film from which the VFFS package is formed often extends into the heat sealed regions of the VFFS package. As a result, the printed ink system that forms the image must be able to withstand the heat applied during the heat seal process, without smearing or otherwise degrading or distorting the appearance properties of the printed image (e.g., gloss). The printed ink system must also withstand the flexing, abrasion, and rub conditions associated with the packaging application. A water or solvent-based ink system applied to the surface of the thermoplastic film (i.e., “face-printed” film) typically will not withstand such exposure. For example, many surface-printed inks melt or stick to the heat seal jaw during the heat-sealing process.
Considerations such as those discussed above with respect to VFFS packaging also exist for: 1) horizontal form-fill-seal (“HFFS”) packaging and 2) packaging that uses a lidding thermoplastic film heat-sealed to a bottom tray, cup, or thermoformed container. These types of packaging applications are well known in the packaging industry. For example, hot dogs are often packaged in a film-lidded thermoformed package having a flexible bottom portion. Meat and poultry is often packaged in a film-lidded foam or other semi-rigid bottom tray. Yogurt and other dairy products are often packaged in a film-lidded rigid cup-like bottom portion.
A suitable “trap-print” film may help prevent the heat-seal distortion of the printed image on the thermoplastic film used in VFFS, HFFS, or lidding applications. A trap-print film sandwiches the printed ink between a substrate film layer and a top film layer that is laminated to the substrate film. As such, the top film helps to protect the printed image from heat distortion and degradation. However, a trap-print film requires the additional manufacture step of laminating the top film to the film substrate, and therefore is generally more expensive and complicated to manufacture.
If a trap-print film is not used, then water- and solvent-based overprint varnishes may be used to cover and enhance the protection of the underlying printed ink image. However, such overprint varnishes are generally based on formulations that are similar to the underlying inks (absent the pigment), and are therefore subject to the same heat and abuse limitations as the underlying printed ink. Further, while such overprint varnish systems may provide enhanced attributes in one or more of the areas of heat resistance, flexibility (i.e., crack resistance), abrasion resistance, and gloss—they have not always provided acceptable attributes in all four areas.
Generally, printing inks and overprint varnishes applied to packaging films in food applications are printed so that the ink or varnish will not directly contact the packaged food product. For example, the ink may be surface-printed on the non-food side, outside (i.e., the side opposite the food-contact side) of the packaging film. Nevertheless, concern exists that one or more components of a surface-printed ink system and/or overprint varnish may migrate through the packaging film to directly contact the packaged food. If a component does migrate to contact the packaged food, then the U.S. Food and Drug Administration (FDA) considers the component an indirect “food additive.” Most printed ink and overprint varnish components and systems are not FDA-approved as either direct or indirect food additives. Accordingly, it is important to establish that each component of a printed ink system for food-packaging films will not reasonably be expected to migrate through the substrate film to contact the packaged food.
To establish that a printed ink or overprint varnish component will not migrate through the printed film in a significant amount, a packager will typically conduct a migration study. Generally, a properly conducted migration study for a printed ink system for a packaging film is one that accurately simulates the condition of actual packaging use—and also uses analytical methods sensitive to the equivalent of 50 parts per billion (ppb). A reliable migration study for a printed packaging film typically involves either forming the film into a package that is filled with a food-simulating solvent (i.e., “food simulant”) or by installing a specimen of the printed film in a migration cell for extraction by the food simulant. The volume of food simulant-to-film surface area should reflect the ratio expected to be encountered in the actual packaging application. The FDA set forth the protocol for obtaining reliable migration data; the FDA migration study protocols are discussed in “Recommendations for Chemistry Data for Indirect Food Additive Petitions,” Chemistry Review Branch, Office of Premarket Approval, Center for Food Safety & Applied Nutrition, Food & Drug Administration (June, 1995), which is incorporated in its entirety by reference. A typical fatty-food simulant for the migration test is 95 weight % ethanol and 5 weight % water. A typical aqueous-food simulant for the migration test is 5 weight % ethanol and 95 weight % water. A representative food simulant-volume to film-surface area is 10 milliliters per square inch. The migration test may be conducted, for example, at 40° C. for 10 days.
Radiation-curable inks and varnishes have had some acceptance in a print system for non-food packaging applications—and also for food-packaging applications that use paper or cardboard carton as the print substrate so that the packaged food either does not directly contact the printed packaging material or the print substrate is so thick that there is no reasonable expectation of migration of the printed components into the food. However, radiation-curable ink systems have not found acceptance for use with relatively thin thermoplastic films in food-packaging applications because of the susceptibility of such a system to unacceptable levels of migration into the packaged food of the unreacted monomers, reaction by-products (e.g., photodegradation products), and/or residual photoinitiator of the radiation-curable ink system.